1. Field of the Invention
The present invention relates in general to a so-called hybrid drive system for driving a motor vehicle, which drive system includes an engine and an electric motor. More particularly, the invention is concerned with a technique for permitting the motor vehicle to run to a desired destination even in the event of a failure of one of the engine and the electric motor where the engine and the electric motor are disposed in parallel connection with vehicle drive wheels, or a failure of one of the engine and an electric generator where the engine and the electric motor are disposed in series connection with the drive wheels.
2. Discussion of the Related Art
JP-A-5-50865 discloses an example of a hybrid drive system of so-called "parallel type", which includes (a) an engine operated by combustion of a fuel, (b) an electric energy storage device which is charged with an electric energy by rotation of the engine, and (c) an electric motor operated by the electric energy stored in the electric energy storage device, and wherein the engine and the electric motor are selectively used to drive the vehicle, depending upon the running condition of the vehicle. This type of hybrid drive system has a low-load drive mode in which only the electric motor is used to drive the vehicle, and a high-load drive mode in which the engine is used to drive the vehicle. The low-load and high-load drive modes are selectively established depending upon the running condition of the vehicle. The low-load drive mode is selected when the load acting on the hybrid drive system is relatively low (namely, when the currently required power or output for driving the vehicle is relatively small), and the high-load drive mode is selected when the load acting on the drive system is relatively high (namely, when the currently required power or output is relatively large). Also known is a hybrid drive system of so-called "series type", which includes (a) an engine operated by combustion of a fuel, (b) an electric generator driven by the engine for generating an electric energy energy, (c) an electric energy storage device for storing the electric energy generated by the electric generator, and (d) an electric motor operated by the electric energy generated by the electric generator or stored in the storage device, and wherein the electric motor is used as a drive power source for driving the vehicle. There is also known a hybrid drive system of so-called "parallel series type" which is a combination of the parallel type hybrid drive system and the series type hybrid drive system, wherein the engine of the series type hybrid drive system is used as another drive power source in addition to the electric motor.
In the event of a failure of the engine or electric motor in the parallel type hybrid drive system, the motor vehicle cannot be driven at all or cannot be driven a distance sufficient to reach the desired destination. If the engine becomes defective, the vehicle should be driven by the electric motor. Since the torque produced by the electric motor is relatively small, the vehicle cannot be driven by the electric motor with a sufficient torque where the required power is relatively large, for instance, where the vehicle is required to climb an uphill. It is also noted that the electric energy is stored in the storage device by the engine as well as by utilizing regenerative braking of the electric motor, the failure of the engine results in an insufficient amount of the electric energy stored in the storage device, making it impossible to operate the electric motor for driving the vehicle over a distance sufficient to reach the desired destination. If the electric motor becomes defective, the vehicle should be driven by the engine. Since the drive system is designed to select the engine drive mode only when the vehicle is in a high-load condition, the vehicle cannot be driven by the engine in the engine drive mode when the vehicle is in a low-load condition, for example, immediately after the vehicle has been started or immediately before the vehicle is stopped. Where the hybrid drive system of the parallel type is equipped with a transmission which is shifted to change its speed ratio depending upon the running condition of the vehicle, a failure of one of the engine and the electric motor causes deteriorated running performance of the vehicle. That is, the power transmitted from the normal drive source (normally functioning engine or electric motor) to the transmission does not match the selected speed ratio if the transmission is shifted to change its speed ratio depending upon the amount of operation of the accelerator pedal or other running condition of the vehicle in the same manner as in the normal state of the drive system. In this case, the vehicle may not be driven at all, or the running distance is considerably limited.
In the event of a failure of the electric motor as the drive power source in the series type hybrid drive system, it is certainly impossible to drive the vehicle. However, a failure of the engine or electric generator also causes a problem. Namely, the failure of the engine makes it impossible to operate the electric generator to generate the electric energy, and the electric energy that can be used by the electric motor to drive the vehicle is limited to that stored in the electric energy storage device. If the vehicle is run under a relatively high load, the running distance is limited particularly since the energy efficiency is relatively low in the high-load running condition. If the ratio of the electric energy necessary to operate the electric motor to the electric energy stored in the storage device is controlled, the failure of the electric generator to generate the electric energy results in shortage of the electric energy to be used by the electric motor, leading to a possibility of running failure of the vehicle due to an insufficient output of the electric motor.